Our overall objectives remain to be the immunological and biochemical characterization of human leukemia-lymphoma (HLL)-associated cell membrane antigens and the preparation of specific anti-HLL antibody reagents for clinical use as well as for immunochemical studies of human malignant lymphoreticular cells. In addition to our previously described monoclonal antibody (mAb) SN1 directed to a unique human T-cell leukemia antigen, termed TALLA, we have generated monoclonal antibodies, termed SN2, SN2a, and SN2b, that define a human T-cell leukemia-associated cell surface glycoprotein, GP37, with an approximate Mr = 37,000. These antibodies were generated by using a human leukemia antigen preparation. The reactivity and specificity of these monoclonal antibodies (mAbs) were characterized by a sensitive radioimmunoassay against a variety of cultured and uncultured human cells. In selected cases, the cell specimens were further tested by indirect immunofluorescence-staining. Among the various cultured malignant and nonmalignant human cell lines tested, these mAbs reacted only with leukemic T-cell lines with one exception. Results consistent with the above were obtained from studies in which uncultured malignant cell specimens from different cancer patients were tested against these mAbs; they reacted only with T leukemia cells. Among various uncultured normal cell specimens tested, these mAbs did not react with thymocytes, bone marrow cells, peripheral blood lymphocytes containing B and T cells, purified T cells, monocytes, granulocytes, or erythrocytes. However, they reacted with platelets. It should be noted that mAb SN1 does not react with platelets. SN2 and SN2a immunoprecipitate a Mr = 37,000 antigen from both a [unreadable]125[unreadable]I-labeled leukemia antigen preparation and from [unreadable]125[unreadable]I-labeled glycoproteins of MOLT-4 cells. These mAbs will be useful in diagnosis and perhaps also in therapy of human T-cell leukemia, which is known to be associated with poor prognosis. It is important to note that immunotoxins prepared by conjugating ricin A chain with mAbs SN1 and SN2 showed specific killing of human T leukemia cells in vitro. (2)